Various technical solutions have been proposed for preparing heating resistors on a substrate. For example, heating elements in the form of a resistance comprising a copper wire, for example, can be bonded to the substrate. However, this solution is unsatisfactory because it requires a long and costly manufacturing process, generating integration constraints, and liable to at least partial detachment of the heating element during heating cycles, associated with the variations in moisture of the environment.
Another solution for replacing the bonding is the electrolytic deposition of resistive materials. However, besides the fact that the level of adhesion obtained is unsatisfactory, it appears that the electrolysis is limited to the deposition of a small number of materials, of which the resistivity is too low to constitute a heating element without preparing a resistive circuit by complex methods of selective deposition, etching or marking.
In view of the above, to solve the problem posed of obtaining high and durable adhesion, at the same time as a wide choice of materials having satisfactory resistivity, it has appeared that vacuum deposition techniques, particularly by cathode sputtering, may provide a satisfactory solution.
Thus U.S. Pat. No. 6,365,483 describes a method for creating a heating system by the vacuum deposition of a resistive material known to a person skilled in the art. The technical solution described in this document is unsatisfactory because it imposes a long and costly process, involving at least one step for preparing a resistive circuit by photolithography technologies in order to obtain the desired resistance of the film.
Another technical problem to be solved appears with regard to the material used, depending on the intended application, when the heating system has to operate in a corrosive environment. This may be the case, for example, for defrosting rear view mirrors of motor vehicles, where the substrate, receiving the reflecting part, can be heated. The resistive film, that is the heating film, must withstand this type of environment. Moreover, the resistive film must advantageously be able to be supplied with 12 V or 48 V voltage, for example, but must not be too thin to be easily controlled by the conventional method, nor too thick, in order to avoid increasing its cost and decreasing its service life for reasons perfectly known to a person skilled in the art (cracking, residual stresses, etc.). This dual constraint, resulting from a predefined thickness on the one hand, and from the need for a power supply voltage on the other, defines an imperative acceptable resistivity range for the choice of the material.
It has appeared that the materials currently available and known for their resistivity properties are unsuitable for achieving these objectives.
Surprisingly and unexpectedly, it has appeared that titanium-copper-nickel based alloys constitute a satisfactory solution for solving the problem posed, since such alloys have a resistivity of between 200 and 300 μohm/cm and also excellent corrosion resistance (above 400 hours in salt mist), according to standard NFX41002.
While such titanium-copper-nickel alloys are perfectly familiar to a person skilled in the art, as a brazing alloy (U.S. Pat. No. 3,651,099, U.S. Pat. No. 3,652,237) usable for its biocompatible properties (U.S. Pat. No. 4,774,953) or as an adhesion promoter in metal matrix composites (U.S. Pat. No. 410,133, U.S. Pat. No. 5,645,747, U.S. Pat. No. 5,530,228), they are not known to have resistivity properties, no publication having reported such properties.
It has also been observed that, contrary to most materials deposited in thin films, this range of titanium-copper-nickel alloys has the particular feature of preserving a constant resistivity, even after several heating-cooling cycles.